[Fixes] Crash in Deciders when pickers are null

src/main/java/org/betterx/bclib/api/v2/generator/BiomeDecider.java

@@ -158,6 +158,8 @@ public abstract class BiomeDecider {
      * Called whenever the picker needs to rebuild it's contents
      */
     public void rebuild() {
+        //TODO: 1.19.3 test if this rebuilds once we have biomes
+        if (picker != null)
             picker.rebuild();
     }